'Sackcloth' in the Bible

The people of Nineveh believed God. They called for a fast and put on sackcloth, from the greatest of them to the least important.

When the message reached the king of Nineveh, he got up from his throne, removed his royal garments, covered himself with sackcloth, and sat down in ashes.

No man or animal, herd or flock, is to taste anything, graze, or drink water. Instead, let both man and animal clothe themselves with sackcloth and cry out to God forcefully. Let every person turn from his evil ways and from his tendency to do violence.